The present invention relates to a continuous process for the preparation of aromatic diisocyanates by phosgenation of the corresponding diamines which is carried out in the gas phase.
Although the preparation of organic isocyanates from the corresponding amines by reaction with phosgene in the gas phase has long been known (e.g., Siefken, Ann. 562, 108 (1949)), the process has, until now, only been of technical importance for monoamines (e.g., Ullmann, 4th Ed. Vol. 13, p. 353) and (cyclo)aliphatic diamines. For instance, (cyclo)aliphatic diisocyanates can be successfully prepared in the gas phase in accordance with EP-A 289 840 at temperatures of 300.degree. to 500.degree. C. in a reaction tube with reaction times of 10.sup.-4 s. Aromatic diamines, on the other hand, have been phosgenated in the liquid phase (e.g., Ullmann, 4th Ed. Vol. 13, p. 351) to produce the corresponding diisocyanates. Gas-phase reaction of aromatic diamines with phosgene has failed due to the formation of solids which blocked up the equipment and reduced the yield of diisocyanate.
The formation of solids can be ascribed to both excessively long and excessively short reaction-times. With short reaction times, isocyanate amino hydrochlorides are formed in addition to polyurea dust. With excessively long reaction times, carbodiimides and isocyanurates are formed.